(Redirected) Graphics card/driver?

Hi. I have an Alienware X51 R2 that I have recently taken apart to install a graphics card or driver. The one I purchased was a Radeon R9 graphics card, but when I went to install it, it did not fit into the system. I'm at a complete loss as to what to do or what I need in order to get the system up and working. If anyone could give me some advice as to what I need to do or buy that would be greatly appreciated. Thanks.